Synthetic Efforts to Investigate the Effect of Planarizing the Triarylamine Geometry in Dyes for Dye-Sensitized Solar Cells

The geometry of a dye for dye-sensitized solar cells (DSSCs) has a major impact on its optical and electronic properties. The dye structure also dictates the packing properties and how well the dye insulates the metal–oxide surface from oxidants in the electrolyte. The aim of this work is to investigate the effect of planarizing the geometry of the common triarylamine donor, frequently used in dyes for DSSC. Five novel dyes were designed and prepared; two employ conventional triarylamine donors with thiophene and furan π-spacers, two dyes have had their donors planarized through one sulfur bridge (making two distinct phenothiazine motifs), and the final dye has been planarized by forming a double phenoxazine. The synthesis of these model dyes proved to be quite challenging, and each required specially designed total syntheses. We demonstrate that the planarization of the triarylamine donor can have different effects. When planarization was achieved by a 3,7-phenothiazine and double phenoxazine structures, improved absorption properties were noted, and a panchromatic absorption was achieved by the latter. However, an incorrect linking of donor and acceptor moieties has the opposite effect. Further, electrochemical impedance spectroscopy revealed clear differences in charge recombination depending on the structure of the dye. A drawback of planarized dyes in relation to DSSC is their low oxidation potentials. The best photovoltaic performance was achieved by 3,7-phenothazine with furan as a π-spacer, which produces a power conversion efficiency of 5.2% (Jsc = 8.8 mA cm–2, Voc = 838 mV, FF = 0.70).


NH2 Br Br
Compound 21 (100 mg, 0.22 mmol) was mixed with NH 4 Cl (104 g, 1.94 mmol) and iron powder (36 mg, 0.65 mmol), before EtOH (2 mL) and water (0.5 mL) were added under nitrogen atmosphere. The reaction mixture was stirred at 78 °C for 4 h, cooled to rt, filtered through celite using ethyl acetate as eluent, and concentrated. This provided 22 as a colorless wax that was carried forward without further purification. tert-butoxide (42 mg, 0.43 mmol) was added to a Schlenk tube, and nitrogen atmosphere was established.

23
Dry 1,4-dioxane (2 mL) was added under nitrogen and the reaction mixture was heated to 110 °C, the reaction was stirred for 18 hours. Upon cooling down to room temperature, an aqueous solution of NH 4 Cl was added (saturated, 5 mL). The aqueous phase was extracted with ethyl acetate (3 x 25 mL), and the combined organic phase was dried with brine and over Na 2 SO 4 . The solvents were removed in vacuo, and the crude product was purified by silica gel column chromatography (n-pentane/CH 2 Cl 2 , 1:1, R f = 0.66) to give compound 23 as a white solid (26 mg, 0.11 mmol, 50% over two steps from 21), mp.  [2,3,4-kl]phenoxazine, 24, was attempted through the following two procedures [3,4] with slight modifications. In our hands the resulting product mixture proved impossible to purify by column chromatography or recrystallization.
NBS-procedure: A solution of compound 23 (20 mg, 0.073 mmol) in chloroform (4 mL) is mixed with Nbromosuccinimide (11.7 g, 0.066 mmol) at 0 °C under exclusion of light, and the mixture was stirred at this temperature for 2 h. The reaction was quenched by addition of sodium sulfite solution (5w%, 10 mL), and the mixture was stirred at room temperature for a further 30 min. After phase separation, the organic phase was washed with water (20 mL), and the aqueous phase was extracted with dichloromethane (3 x 20 mL).

S23
The combined organic phases were dried over sodium sulfate and concentrated in vacuum. The residue was dissolved in ethyl acetate and filtered over silica gel. The crude product was subsequently recrystallized from hexane.
Br 2 -Procedure: A stirred solution of compound 23 (50 mg, 0.18 mmol) in acetic acid (2 mL) was flushed with nitrogen then a solution of bromine (1 M, 0.20 mL, 1 mmol) in acetic acid was added dropwise, and stirring was continued overnight. The reaction was quenched by addition of sodium sulfite (5w%, 20 mL) solution, and the mixture was stirred at room temperature for a further 30 min. After phase separation, the organic phase was washed with water (20 mL), and the aqueous phase was extracted with dichloromethane (3 x 20 mL).